Large power stations with outputs in the range of more than 100 MW in which a current-generating generator is driven by a gas turbine and/or steam turbine and feeds the generated electrical output into an electrical grid with given grid frequency (e.g., 50 or 60 Hz) typically have a fixed coupling between the (mechanical or aerodynamic) rotational speed of the turbine and the grid frequency. The output of the generator is connected by a grid connection at a locked frequency to the electrical grid, while it is driven by the turbine either directly (single-shaft arrangement) or via a mechanical gearbox coupled in terms of rotational speed. Such configurations of power stations are illustrated in a greatly simplified form in FIGS. 2 and 3. Only fixed transmission ratios between the grid frequency and turbine can be realized when gearboxes are utilized. However, solutions are also conceivable in which the generator is driven by a power turbine that can be driven with a rotational speed deviating from that of the actual gas turbine.
In a greatly simplified diagram, FIG. 1 shows a power station 10′ of known type that generates current by a gas turbine 12 coupled to a generator 18 and feeds it into an electrical grid 21. The gas turbine 12 and the generator 18 are connected by a common shaft 19 and form a single-shaft turbine shafting 11. In the simplest case, the gas turbine comprises a compressor 13 that draws in and compresses combustion air by an air inlet 16. The compressor 13 can be composed of several sub-compressors that are connected one behind the other and that work at increasingly greater pressure levels and optionally allow intermediate cooling of the compressed air. The combustion air compressed in the compressor 13 is led into a combustion chamber 15 into which liquid fuel (e.g., oil) or gaseous fuel (e.g., natural gas) is injected by a fuel feed 17 and combusted under the consumption of combustion air.
The hot gases discharged from the combustion chamber 15 are expanded in a subsequent turbine 14 under work and thereby drive the compressor 13 and the connected generator 18. The exhaust gas that is still relatively hot at the outlet of the turbine can also be sent through a subsequent heat recovery steam generator 23, in order to generate, in a separate water-steam circuit 25, steam for the operation of a steam turbine 24. Such a combination is designated as a combined cycle power station. The steam turbine 24 can be coupled with the generator 18 on the side opposite the turbine 14. However, it can also drive a separate generator.
In the single-shaft arrangement of FIG. 1, the rotational speed of the gas turbine 12 is at a fixed ratio with respect to the frequency of the AC voltage that is generated in the generator 18 and that must be equal to the grid frequency of the electrical grid 21. For large gas-turbine units that are typical today with outputs greater than 100 MW, a rotational speed of the gas turbine of 3600 rpm (e.g., gas turbine Model GT24 by the Assignee of the present application) is allocated to the generator frequency or grid frequency of 60 Hz and a rotational speed of 3000 rpm (e.g., gas turbine Model GT26 by the Assignee of the present application) is allocated to the generator frequency of 50 Hz.
If a different ratio is to be achieved between the rotational speed of the gas turbine 12 and the generator or grid frequency, then a mechanical gearbox 26 that is typically formed as a reducing gearbox and thus allows higher rotational speeds and smaller constructions of the gas turbine 12 can in principle be inserted, according to FIG. 2 in a power station 10″, between the shaft 19 of the gas turbine 12 and the generator 18 (turbine shafting 11′). Such mechanical gearboxes 26, however, can be used only for outputs up to 130 MW for reasons of stability. On the other hand, large outputs for each gas turbine of greater than 100 MW and high degrees of efficiency are achieved, above all, with relatively low-speed single-shaft machines.
The following disadvantages result from the rigid coupling between the turbine rotational speed and the grid frequency:                A stable operation on the electrical grid is possible only to a limited extent.        It leads to output drop-offs in the turbine or to thermal and mechanical loading in the dynamic control for grid-frequency support by raising the gas-turbine inlet temperature.        Grid frequency-independent or load-independent output control of the power station is not possible.        Grid frequency-independent or load-independent efficiency optimization of the power station is not possible.        Grid frequency-independent or load-independent partial-load optimization of the power station is not possible.        Emissions control of the gas turbine is possible only to a limited extent.        Conventionally, in the case of an under-frequency event, the power station first loses output; only after equalization of the insufficient output through corresponding readjustment can the power station actively support the electrical grid through excess output. Conversely, in the case of an over-frequency event, the power station first increases output; only after equalization of the excess output through corresponding readjustment can the power station actively support the electrical grid through output reduction.        Transients are produced for fluctuations in frequency:                    In the case of an under-frequency event, as the first step, the intake mass flow is reduced; this leads (at first for constant fuel mass flow) to over-firing and usually shortly thereafter to under-firing due to corrective action.            Analogously, in the case of an over-frequency event, there is under-firing followed by over-firing.            These transients lead to reduced service life and increased emissions (NOx in the case of over-firing and CO in the case of under-firing).            For frequency support, in the case of an under-frequency event, the hot-gas temperature is increased with the disadvantageous consequences of reduced service life of the components as well as increased NOx emissions.                        
From U.S. Pat. No. 5,694,026, a single-shaft turbine generator set without a step-down gear is known, in which a static frequency converter is arranged between the output of the generator and the electrical grid, wherein, with the help of this frequency converter, the AC voltage frequency generated by the generator is converted to the frequency of the electrical grid. When the gas turbine is started, the generator is used as a motor that is supplied with energy from the electrical grid via the static frequency converter.
From U.S. Pat. No. 6,979,914, a power station with a single-shaft arrangement made from a gas turbine and generator is known in which a converter is similarly provided between the generator output and the electrical grid, in order to adapt the AC voltage generated by the generator to the grid frequency.
From the article by L. J. J Offring a, et al. “A 1600 kW IGBT Converter With Interphase Transformer For High Speed Gas Turbine Power Plants,” Proc. IEEE-IAS Conf. 2000. 4, 8-12 Oct. 2000, Rome, 2000, pp. 2243-2248, a power station with a high speed gas turbine (18,000 rpm) and comparatively smaller output power (1600 kW) is known, in which frequency decoupling between the generator and electrical grid is realized by a converter.
Special control and operating concepts for supporting the grid frequency, however, are not to be taken from these publications.